1. Field of the Invention
The present invention relates to a structure for connecting a power semiconductor device and a printed wiring board to hold the power semiconductor device thereon.
2. Description of the Background Art
Regarding a power semiconductor device such as a power module, connection with small power loss between the power semiconductor device and an external printed wiring board, and efficient dissipation of heat generated from a power semiconductor element in the power semiconductor device to the outside, are believed to be essential requirements to control high current and high voltage. So, reduction in resistance in the power semiconductor device and of each wiring pattern on a printed substrate, reduction in connection resistance at each connecting part, and improvement in connection reliability have been important issues to be achieved.
Meanwhile, in terms of simplification of assembly, various techniques to easily and reliably connect a power semiconductor device and a printed wiring board have been suggested. As an example of such techniques, Japanese Patent Application Laid-Open No. 2001-298129 (patent literature 1) suggests a structure using a wire pin as an external terminal of a power semiconductor device and which projects from a surface of the power semiconductor device. In this structure, connection between a substrate (internal substrate) in the power semiconductor device and the wire pin is formed by inserting the wire pin into a metallic tubular member (bushing) formed on the internal substrate. Connection between the wire pin and an external printed wiring board is established by inserting the wire pin into a through hole in the printed wiring board, and by soldering (through-hole connection system).
Japanese Patent Application Laid-Open No. 2008-198597 (patent literature 2) employs a press contact system to establish connection between a power semiconductor device and a printed wiring board to simplify work of connection. Patent literature 2 uses a spring member (contact spring) formed by bending metal as an external terminal of the power semiconductor device.
In the structure of patent literature 1, the wire pin as an external terminal of the power semiconductor device is inserted into the bushing on the internal substrate, and is held by the force of friction between the wire pin and the inner surface of the bushing. This may result in failure to achieve connection reliability at a satisfactory high level between the internal substrate of the power semiconductor device and the external terminal. Further, use of the through-hole connection system to establish connection between the external terminal and the printed wiring board necessitates a step of defining a through hole during formation of the printed wiring board, and a soldering step when the power semiconductor device is mounted to the printed wiring board.
With regard especially to a power semiconductor device that controls high current, the need to increase the number of wire pins involves increase in the number of through holes and the number of parts to be soldered, leading to cost increase. The power semiconductor device controlling high current generates a great amount of heat. So, stress due to difference in heat expansion coefficient between the printed wiring board and wire pins may generate a crack in solder. Generation of a crack increases connection resistance between the wire pins and the printed wiring board and reduces connection strength, leading to a fear of reduction in reliability.
Patent literature 2 uses a contact spring as an external terminal of the power semiconductor device. The structure of the contact spring itself makes a contact area small with the wiring pattern of the internal substrate, or with a pad part of the printed wiring board, resulting in small current capacity of each external terminal. This increases the number of contact springs required in the power semiconductor device controlling high current, putting an obstacle to size reduction of the device. The contact spring extends a current flow path as it is formed by bending metal, disadvantageously resulting in large power loss.